This invention relates to a method for manufacturing NbO electrolytic capacitors, and capacitors formed thereby, with improved performance and an extended range of working voltages.
Electrolytic capacitors containing anodes made from niobium oxides and sub-oxides (typically NbO) were first described by James Fife in U.S. Pat. No. 6,416,730. These anodes were made by placing a lead wire into NbO powder and pressing the powder into a pellet. The pellets were then sintered, preferably, in a vacuum. The lead wire, which forms the anode termination, is typically made of Ta or Nb wire. Subsequent steps of manufacturing of NbO capacitors are similar to the Ta capacitors. They include formation of dielectric film by anodizing the NbO pellet, impregnation of the formed anode with a MnO2 cathode, top-coating with carbon and silver, assembly, and testing. The advantages of NbO capacitors relative to Ta capacitors include availability of raw materials and a non-burning failure mode. The non-burning failure mode is believed to be due to the fact that NbO has 50% oxygen, stoichiometrically, which makes the igniting energy much higher than that of Ta. Furthermore, after electrical breakdown NbO capacitors have 100 ohm to 1000 ohm residual resistance while Ta and Nb capacitors are typically shorted after electrical breakdown. In comparison to wet Al capacitors, NbO capacitors have higher volumetric efficiency, higher reliability, lower ESR, and better thermal stability of AC characteristics.
The disadvantages of NbO capacitors versus Ta capacitors are lower working voltages, higher DC leakage, lower volumetric efficiency, and higher ESR for a given case size. Higher ESR is caused by lower conductivity of NbO in comparison to conductivity of typical metals like Ta, Nb and Al.
In the prior art, where either Ta or Nb wire are used as the anode lead, oxygen diffusion from NbO powder into the wire takes place during sintering of NbO powder. The driving force for this diffusion is a large difference in oxygen content between NbO, which is about 50% atomic oxygen, and Nb or Ta, which is below about 2% atomic oxygen. Oxygen diffusion increases exponentially with temperature and is very active at the sintering temperature of NbO powder which may exceed about 1200° C. As a result of this diffusion, Ta or Nb wire becomes enriched with oxygen. Adjacent to the wire area NbO powder becomes depleted of oxygen. Calculations described in B. Boiko, Y. Pozdeev, et al., Thin Solid Films, 130 (1985) 341, using oxygen diffusion parameters, suggest that depletion of oxygen in NbO powder during sintering results in its transformation into Nb saturated with oxygen in a layer about 10 μm thick surrounding the wire.
The amorphous dielectric film formed on oxygen saturated Nb is highly susceptible to crystallization (Y. Pozdeev Freeman, Mat. Res. Symp. Proc., 788 (2004) 109). When crystals grow in an amorphous matrix of dielectric they eventually disrupt the dielectric field due to mechanical stress caused by differences in specific volume of amorphous and crystalline phases. As a result of disruption of the dielectric, DC leakage increases. Thicker dielectrics in higher working voltage capacitors suffer more from the crystallization which limits the working voltage available in NbO capacitors. Furthermore, higher working voltages require coarse powder and higher sintering temperature. The latter stimulates growth of the oxygen saturated Nb layer around the wire, which makes crystallization of the dielectric even more severe.
Through diligent research the present inventors have developed a method for mitigating the deficiencies of a NbO capacitor thereby allowing the anticipated advantages to be fully exploited. The present invention solves a long standing problem in the art which was previously not understood and for which a solution was lacking.